Recently, an image forming apparatus of an electrophotographic system such as a laser printer, a digital copying machine, or a laser facsimile includes a post-process apparatus (a finisher) that staples a sheet bundle. The finisher includes a stapler for stapling the sheet bundle. The finisher in the past includes, on an upstream side in a sheet conveying direction of a process tray, standby trays for temporarily storing sheets. The standby trays temporarily store (buffer) one to several sheets while the finisher staples sheets on the process tray. The standby trays temporarily store (buffer) one to several sheets while the finisher sorts sheets on the process tray.
However, when the finisher puts sheets on standby on the standby trays, it is also likely that the finisher further puts another sheet on standby on top of a sheet already put on standby on the standby trays. When friction between the sheet already put on standby on the standby trays and the sheet put on standby on top of the sheet is large or when a contact angle between the sheets is large, a paper jam occurs.